In electrophotographic machines, a photoconductive material receives a relatively uniform charge level which may be, for example, 800 volts. Since the photoconductive material is an insulator, that charge remains on the material until it is exposed to light. When exposed, the exposed portion of the photoconductor is discharged to a level which can approach zero volts. In the electrophotographic process, the exposed photoconductor is then developed and a print is made from the developed image.
Print quality in an electrophotographic machine deteriorates over a period of time as the electrostatic properties of the photoconductive material change with age. Also, repeated use of the photoconductor can change surface properties and affect print quality. For example, repeated application of developing material can film the surface and make it less receptive to light. Additionally, since print paper is pressed against the photoconductor during the transfer of the image to the paper, the surface of the photoconductor is roughened and darkened. These factors add together to change the charge and light receptivity of the photoconductor and eventually necessitate its replacement.
In order to improve and maintain print quality over the life of the photoconductor, electrostatic probes are placed near the photoconductive surface to sense changes in electrostatic charge level. When significant changes in charge levels are sensed, either the charge level, the illumination intensity, or the developer bias level can be adjusted in order to compensate for the changes. See, for example, the system described in U.S. Pat. No. 4,326,796.
Despite the advantages which are theoretically achievable from the use of an electrostatic probe in an electrophotographic machine, such probes have not come into common use. Reasons for that include the expense of the probe and its associated circuitry together with relatively poor sensing capabilities which limit the value of the measurement. This invention seeks a solution to these problems by providing a probe of unusual and unique construction which combines several functions into a single piezoelectric bender element thus reducing the cost of the probe itself and enabling a probe of improved sensing capability.
When probes are used to measure the charge on an electrostatic surface, an initial requirement is to establish a reference to calibrate the significance of the measured quantity. In electrophotographic machines which have a conductive drum seal breaking the continuity of the photoconductive surface, the charge level of the drum seal can provide the needed reference since such a seal is grounded or at some other known potential. In other electrophotographic machines where the photoconductive surface is continuous, chopping schemes are often used to momentarily shield the probe from the unknown voltage on the photoconductive surface and replace it with a reference voltage. The probe of this invention utilizes a different scheme, a nulling arrangement where the photoconductor voltage is successively compared to a known voltage which is varied until the two are equal. Schemes of this type utilize a vibrating sense electrode placed between a surface of unknown potential (the photoconductor) and another surface of known potential. As the electrode vibrates, the capacitance between the unknown potential and the sense electrode changes to generate a current in the electrode. That current is used to increase or decrease the potential on the known surface until it equals the photoconductor voltage. At that point, the vibrating electrode output is reduced to zero.
Vibrating probes generally have three elements, a drive element, a sense element, and a feedback element. Tuning forks are usually used as the vibrator and have a magnetic or piezoelectric element attached as a driver to set the tuning fork into oscillation. A sense element, a conductor of some kind, is attached to the tuning fork, and a feedback element is often attached in order to maintain the frequency of the tuning fork at resonance. Such probes are difficult to manufacture and are expensive. The object of this invention is to eliminate bulky apparatus such as tuning forks to enable close positioning of the vibrating probe to the surface to be measured in order to improve probe sensitivity and reduce its cost.